1. Field of the Invention
The present invention is directed to distillation. It has particular, but not exclusive, application to using rotary heat exchangers to purify water by distillation.
2. Background Information
One of the most effective techniques for purifying water is to distill it. In distillation, the water to be purified is heated to the point at which it evaporates, and the resultant vapor is then condensed. Since the vapor leaves almost all impurities behind in the input, feed water, the condensate that results is typically of a purity much higher in most respects than the output of most competing purification technologies.
One of the distillation approaches to which the invention to be described below may be applied employs a rotary heat exchanger. Water to be purified is introduced to one, evaporation set of heat-exchange surfaces, from which the liquid absorbs heat and evaporates. The resultant water vapor is then typically compressed and brought into contact with another, condensation set of heat-exchange surfaces that are in thermal communication with the set of evaporation heat-exchange surfaces. Since the water vapor on the condensation side is under greater vapor pressure than the water on the evaporation side, vapor that condenses on the condensation side will be hotter than the evaporating liquid on the evaporation side, and its heat of evaporization will therefore flow to the evaporation side: the system reclaims the heat of evaporization used to remove the relatively pure vapor from the contaminated liquid. To minimize the insulating effects to which a condensation film on the condensation surfaces would tend to contribute, a rotary heat exchanger""s heat-exchange surfaces rotate rapidly, so the condensate experiences high centrifugal force and is therefore removed rapidly from the condensation surfaces.
Typically, not all or even most of the liquid that is sprayed onto the evaporation surfaces evaporates in the first pass. Instead, some way of collecting liquid that has passed through the evaporation chambers without evaporating is provided, and the liquid collected by the collector, or xe2x80x9csump,xe2x80x9d is recirculated, to be reintroduced to the evaporation chamber for further evaporation. So the overwhelming majority of the liquid introduced into the evaporation chambers is liquid that has been thus recirculated; unrecirculated feed liquid from the distiller""s inlet is introduced only at a rate slightly greater than that rate necessary to make up for evaporation.
Now, evaporation removes substantially pure water, leaving impurities behind, so the impurity concentration within the recirculated water tends to increase with time. There is often a point at which such concentration increases tend to impair distiller operation by, for instance, depositing salts on the heat-exchange surfaces and thereby reducing heat-exchange efficiency, or by raising the evaporation temperature slightly and thereby reducing the heat-transfer rate. For this reason, a percentage of the collected liquid is usually bled from the collector and discharged from the unit as concentrate. This means that the rate of feed-liquid input actually needs to be great enough to make up for not only evaporation but also for concentrate removal. This beneficially limits the sump-liquid concentration to an acceptable level.
Although concentrate removal is a necessity, it detracts from distiller output; the more concentrate removed, the lower the purified condensate is for a given input feed rate.
I have invented a way of reducing the rate of concentrate removal required to limit the effective sump concentration to a given effective value. Rather than bleed the concentrate from the sump in a more or less continuous fashion, a distiller that employs the present invention""s teachings drains most or all of the concentrate from the recirculation liquid during only relatively short periods, during which the evaporation chambers are typically irrigated with mostly unrecirculated feed water. Except for those short periods of rapid concentrate removal, the sump liquid is recirculated in the normal manner, but preferably without bleeding it away as concentrate. The result is that the effective, average sump concentration tends to be lessxe2x80x94down to little more than halfxe2x80x94the concentration of the concentrate discharged. So the rate of concentrate removal for a given effective sump concentration can be significantly less than conventional concentrate-removal techniques would require.